Electrical transmission and distribution equipment is subject to voltages within a fairly narrow range under normal operating conditions. However, system disturbances, such as lightning strikes and switching surges, may produce momentary or extended voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions. These voltage variations often are referred to as over-voltage conditions.
If not protected from over-voltage conditions, critical and expensive equipment, such as transformers, switching devices, computer equipment, and electrical machinery, may be damaged or destroyed by over-voltage conditions and associated current surges. Accordingly, it is routine practice for system designers to use surge arresters to protect system components from dangerous over-voltage conditions.
A surge arrester is a protective device that commonly is connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert over-voltage-induced current surges safely around the equipment, and thereby protect the equipment and its internal circuitry from damage. When exposed to an over-voltage condition, the surge arrester operates in a low impedance mode that provides a current path to electrical ground having a relatively low impedance. The surge arrester otherwise operates in a high impedance mode that provides a current path to ground having a relatively high impedance. The impedance of the current path is substantially lower than the impedance of the equipment being protected by the surge arrester when the surge arrester is operating in the low-impedance mode, and is otherwise substantially higher than the impedance of the protected equipment.
Upon completion of the over-voltage condition, the surge arrester returns to operation in the high impedance mode. This prevents normal current at the system frequency from following the surge current to ground along the current path through the surge arrester.
Conventional surge arresters typically include an elongated outer enclosure or housing made of an electrically insulating material, a pair of electrical terminals at opposite ends of the enclosure for connecting the arrester between a line-potential conductor and electrical ground, and one or more other electrical components that form a series electrical path between the terminals. These components typically include a stack of one or more voltage-dependent, nonlinear resistive elements that are referred to as varistors. A varistor is characterized by having a relatively high resistance when exposed to a normal operating voltage, and a much lower resistance when exposed to a larger voltage, such as is associated with over-voltage conditions. In addition to or in place of varistors, a surge arrester also may include one or more spark gap assemblies housed within the insulative enclosure and electrically connected in series with the varistors. Some arresters also include one or more electrically-conductive spacer elements coaxially aligned with the varistors and gap assemblies.
For proper arrester operation, contact must be maintained between the components of the stack. To accomplish this, it is known to apply an axial load to the one or more elements of the stack. Good axial contact is important to ensure a relatively low contact resistance between the adjacent faces of the elements, to ensure a relatively uniform current distribution through the elements, and to provide good heat transfer between the elements and the end terminals.
One way to apply this load is to employ springs within the housing to urge the one or more stacked elements into engagement with one another. Another way to apply the load is to encase the stack of one or more arrester elements in glass fibers so as to axially-compress the elements within the stack. For bonded disk stacks or monolithic disks with a sufficiently high rating, such as, for example, a rating greater than 6 kV, these methods are usually sufficient to sustain a static mechanical load but may not be sufficient to withstand the thermo-mechanical forces experienced by the one or more elements during a high current impulse such as, for example, a 100 kA impulse.
When the bonded disk stack or monolithic disk with a sufficiently high rating, such as, for example, a rating greater than 6 kV, is subjected to a high current impulse, the resulting thermo-mechanical forces tend to cause cracking of the surge arrester elements, which tend to crack in mid-plane when subjected to the thermo-mechanical forces of a high current impulse. For bonded disk stacks of more than one element, there also may be cracking near the center of the bonded disk column. The tendency of an element to crack during high current impulses limits the size of an individual surge arrester element as well as the overall length of a stack of bonded surge arrester elements. There generally is a height-diameter ratio where a monolithic disk or a bonded disk stack will be subject to thermo-mechanical failure due to a high current impulse, typically in the form of a crack at the mid-plane.